Primary column In a typical
U–Pb geochronology analytical mode, a beam of (O2)1− primary ions are produced from a high-purity oxygen gas discharge in the hollow
Ni cathode of a
duoplasmatron. The ions are extracted from the plasma and accelerated at 10 kV. The primary column uses
Köhler illumination to produce a uniform ion density across the target spot. The spot diameter can vary from ~5 μm to over 30 μm as required. Typical ion beam density on the sample is ~10 pA/μm2 and an analysis of 15–20 minutes creates an ablation pit of less than 1 μm.
Sample chamber The primary beam is 45° incident to the plane of the sample surface with secondary ions extracted at 90° and accelerated at 10 kV. Three quadrupole lenses focus the secondary ions onto a source slit and the design aims to maximise transmission of ions rather than preserving an ion image unlike other ion probe designs. A Schwarzschild objective lens provides reflected-light direct microscopic viewing of the sample during analysis.
Electrostatic analyzer The secondary ions are filtered and focussed according to their kinetic energy by a 1272 mm radius 90°
electrostatic sector. A mechanically-operated slit provides fine-tuning of the energy spectrum transmitted into the magnetic sector and an electrostatic quadrupole lens is used to reduce aberrations in transmitting the ions to the magnetic sector.
Magnetic sector The electromagnet has a 1000 mm radius through 72.5° to focus the secondary ions according to their mass/charge ratio according to the principles of the
Lorentz force. Essentially, the path of a less massive ion will have a greater curvature through the magnetic field than the path of a more massive ion. Thus, altering the current in the electromagnet focuses a particular mass species at the detector. r=\sqrt{2Vm \over zB^2} where
r is the radius of curvature of the path,
V is the ion-accelerating potential difference,
m is the mass of the ion,
z is the charge of ion and
B is the strength of the magnetic field. -->
Detectors The ions pass through a collector slit in the focal plane of the magnetic sector and the collector assembly can be moved along an axis to optimise the focus of a given isotopic species. In typical U-Pb zircon analysis, a single secondary
electron multiplier is used for ion counting.
Vacuum system Turbomolecular pumps evacuate the entire beam path of the SHRIMP to maximise transmission and reduce contamination. The sample chamber also employs a
cryopump to trap contaminants, especially water. Typical pressures inside the SHRIMP are between ~7 × 10−9 mbar in the detector and ~1 × 10−6 mbar in the primary column (with oxygen duoplasmatron source).
Mass resolution and sensitivity In normal operations, the SHRIMP achieves
mass resolution of 5000 with sensitivity >20 counts/sec/ppm/nA for lead from zircon. ==Applications==